+654 Let an integer be bad if there is no way to insert a colon among the digits of to form a time of day (on a twelve-hour, digital clock), and good if a colon can be inserted to form a time. For example, 100 is good because we can write 1:00, but 164 is bad. How many bad integers between 100 and 200 are equivalent to 3 mod 4?
+981 Hi supermanaccz!
So from 100 to 200, we first need to determine what numbers are bad.
From 100 to 159, they are all good, since we can have 1:00 and 1:59.
But from 160 to 199, they are all bad. There is no 1:60, since that will be 2 o'clock.
There is no 1: 99.
So we need to calculate how many numbers from 160 - 199 are equal to 3 mod 4.
The smallest number that is from 160 - 199 that is equal to 3 mod 4 is 163.
3 / 4 = 0 R 3
163 / 4 = 40 R 3
The largest is 199.
199 / 4 = 49 R 3
To calculate how many numbers we want from 163 to 199, we do:
\((199-163)/3+1=\boxed{13}\)
That will be your final answer .
I hope this helped,
gavin.
